Process for the heat hardening of pellets composed of ores on sintering grates



em "1- K PROCESS FOR THE HEAT HARDENING OF PELLETS COMPQSED OF ORES NSHNTER- ING GRATES Karl-Heinz Boss, Bad Homburg vor der Hohe, HansRausch, Oberursel (Tauuus), and Kurt Meyer, Frankfurt am Main, Germany,assignors to Metallgesellschaft Aktiengesellschaft, Frankfurt am Main,Germany No Drawing. Filed Feb. 15, 1960, Ser. No. 8,483 Claims priority,application Germany Mar. 25, 195

1 Claim. (Cl. 755) This invention relates to the heat hardening of greenpellets composed of iron ore and, in particular, is directed to thehardening of the pellets by means of hot gases.

It is known to convert fine-grained ores, particularly iron ores, intolumpy form by sin'tering them into cohesive cakes on a sintering band,if necessary, after previously crumbling and/ or granulating the oresand to subsequently break up these cakes into pieces of suitable size.The heat required for carrying out this procedure can be supplied eitherby hot gases or solid fuel and also by a mixture of gas and solid fuel.It is likewise known to fornrfinegrained ores moistenedwith Water intolarge spherical granular bodies with a diameter of 8 to 20 mm. in agranulating drum or on a granulating disc and then to hard-burn thesebodies in a shaft furnace into non-cohesive pellets or balls with theaid of hot gases. -It has also been proposed to apply this process to amoving grate or sintering band. In this case, the moist pellets,so-called green pellets, are charged onto the sintering band, afterfirst preferably laying a protecting grate layer of finished burntmaterial on the band, then running the pellets under a gas dome whererelatively warm gases are drawn through the bed of pellets to predry andpreheat the pellets, then treating the bed with strong oxidizing gasesunder a second gas dome where the actual hard-burning is carried out,and finally cooling the bed in a third zone by cold air sucked throughthe charge The pellets therefore travel successively through threehorizontal zones with different gas atmospheres and at differenttemperatures arranged one following the other, as disclosed in thepatent to Lellep, No. 2,750,272.

In this procedure, however, objections and difficulties are encounteredwhich do not occur in the case of the conventional sintering of ore intocohesive sinter cakes.

As the raw relatively fragile green pellets are sensitive both tomechanical stressing and also to fluctuations in temperature, it isnecessary to introduce various precautionary measures which have adetrimental effect both on the throughput capacity of the process aswell as on the quality of the burnt product obtained. First, it is notpossible to obtain as uniform and level a charge bed as in the case ofthe conventional sintering. Whereas in the conventional sinteringprocess, the charge bed can be smoothed flat with a scraper arranged ata suitable height above the sintering band. This is not possible whenburning green pellets, because the mechanically sensitive green pelletsare seriously damaged and even to a great extent almost completelydestroyed by the scraper. It has therefore hitherto been necessary totry to produce an absolutely level and uniform bed, which howeverresults in that the lowermost portions of the bed are not heated to asufficiently high temperature at the places where the pellets are piledup too high, whereas overheating of the grate cannot be avoided at theplaces where the pellets are not laid to a suflicient'thickness.Furthermore, the thermal sensitivity of the green pellets requires verycareful and consequently time-wasting drying and preheating treatment,before the actual burning process takes place, by

3,02d,l47 Patented Feb. 6, 1962 passing moderately warm gases at about200 to 400 C. through the bed. The suction area of the grate to be madeavailable for the first or preheating zone must therefore be in the sameorder of magnitude as the area of the actual burning zone in which gasesat about 1100 to 1400 C. are drawn through the bed. It is understandablethat, on this account, the total output of the sintering band expressedin tons of throughput per square meter of sintering surface and unit oftime, for example a 24 hour day, is decreased considerably.

This invention enables both of the objections to the known burningprocess to be overcome simultaneously.

This invention has discovered that a lowering of the gas suctionpressure in the preheating zone to reduce the velocity of the flow ofgas through the bed 01' pellets is sufficient in of itself to preventthe pellets from bursting even if the preheating in its previous sensewere dispensed with and the green pellets treated initially andimmediately with hot gases at about 1,000 C. and above. The inventionhas the advantages of simplicity and economy.

Thus the pellets are preheated for a short period of a few minutes witha gas having a temperature of about 1,00G C. or more which ispractically identical with the pellet burning temperature. In drawingthe gas through the bed of pellets at a lower speedin the preheatingzone than in the hard burning zone, a pressure difference between theupper and lower surfaces of the bed amounting to about 20 to 60 mm.water of column (W.col.) is preferably used in the preheating zone andof 70 to 200 mm. W.col. in the hard burning zone.

Whereas, in the prior art processes for the hard burning of greenpellets on a sintering band, the preheating zone constitutes a zoneentirely separate from the actual burning zone in the direction oftravel of the sintering band; the process of this invention enables theburning zone to overlap the drying zone to a considerable extent andconsequently a better utilization is made of the expensive sinteringgrate surface area, with the result that a throughput of, for example,19 to 21 metric tons per square meter of grate surface area in 24 hoursare possible with a certain kind of ore, as compared with 10 to 11 tonsper square meter in 24 hours for the same kind of ore without using theprocess of this invention.

In a further embodiment of this invention, an additional layer composedof perhardened or finished burnt pellets or graded pellet scrap is laidon top of the bed of green pellets on the sintering band. This layer canbe spread without difficulty from a distributing bunker upon the bed ofgreen pellets.

It has been found that this additional layer has sufficient mechanicalstrength to be smoothed by a scraper without the hard pellets or pelletscrap becoming damaged or destroyed. Moreover, the forces produced bythe scraper are widely distributed so that the forces reaching the bedof raw green pellets are so'small as to leave the green pelletspractically undamaged.

By this means therefore, it is possible to form a bed on the sinteringband which is just as uniform and smooth as in the case of conventionalsintering.

' The advantages of the method of this invention are shown by acomparison of the following specific examples. Example 1 is a testcarried out according to the known state of the prior art. Example 2shows that an increase in output cannot be attained by increasing thetemperature of the gas during the green pellet drying period underprocess conditions unchanged from the prior art. Example 3 is thesimplest form of carrying out the method of this invention. Example 4 isan improvement of the method of this invention which consistssubstantially in that, after shutting off the heating, still relativelyhot gases, such as preheated air, are drawn through the bed with theresult that the sensible heat of the upper and middle layers is carriedto the lowermost layer so that this heat content can be renderedavailable for hard-burning the pellets located in the lowermost layers.Examples 5 and 6 correspond with Examples 3 and 4 with the difierencethat an additional cover layer of preburnt pellets is spread upon thegreen pellets. This modification is recommended particularly when thegreen pellets consist of a material which is so sensitive that even whenthe gas vacuum is re duced according to this invention the green pelletsdo not Withstand the direct contact with the hot gases at about 1000 C.without bursting. Instead of the additional layer being composed ofpreburnt pellets, pellet scrap, that is fragments of finished burntpellets constituting more or less waste material, can be used inpractice just as successfully. I

The consumption figures concerning the quantities of heat used per kg.of pellets and obtained from the examples should only be considered asfigures for comparing the examples with one another, because theserepresent values of the heat consumption without furtherrecovery ofwaste heat, such as is customary in practical working. Therefore, allthe comparison figures of the examples, that is of the examples carriedout according to the prior art, as wellas those representing theinvention, are not inconsiderably less favorable than the valuesobtainable with the same methods in practical working.

EXAMPLE 1 State of the prior art 70 kg. of raw green pellets ofmagnetite containing about 64% Fe and 8% SiO with a water content ofabout 9% and an average diameter of to mm. are spread to a depth of 33cm. on a sinter grate having a surface area of x 30 cm. after the metalgrate has been first covered with a protective grate layer of burntpellets to a depth of 10 cm. The total thickness of the layer is then33+l0=43 cm. A side layer of broken pellets with a grain size of 2 to 10mm. is then poured between the green pellets and the side of the grate.In order to harden the green pellets by heat treatment, the grate iscovered above the bed of green pellets with a hood in which a burner islocated. The hardening of the pellets is carried out in the followingstages:

(a) Drying the green pellet.-For a period of 15 minutes gases with atemperature of 300 C. are sucked through the charge at a vacuum of 80 to100 mm. water column.

(b) Heating up.-During the next 13 minutes, the tem-' perature of thegases sucked through the charge is increased from 300 to 1100 C. at avacuum of 100 to 150 mm. water column.

(0) Burning.-For the burning operation, gases at a temperature of 1250C. are sucked through the charge for 14 minutes until the pellets abovethe protective grate layer have reached this temperature. Then thecharge is allowed to stand for 5 minutes without suction being applied.

(d) C0'0ling.For 25 minutes, cooling air is forced through the gratefrom the bottom towards the top.

The pellets thus produced have an average strength of 300 to 400 kg. perpellet. The output of the sintering grate per square meter suction areaand 24 hours is approximately 13 to 14 metric tons of finished pellets.During the different stages of the method, between 7 and 7.7 cubicmeters of gas per minute were sucked through the charge. The heatconsumption, calculated without beneficiation of the heat content of thefinished product, was 1350 kcal./kg. burnt pellets.

EXAMPLE 2 With the'same charge of green pellets on the sinter grate asin Example 1, the temperature of the drying gases at the same vacuum of80 to 100 mm. water column is immediately increased to 1000 C. After afew minutes, the test had to be stopped. The reason for this was thepremature bursting of the green pellets in the upper layers which causeda reduction in the porosity of the bed charge and the stoppage of thepassage of gas through the bed.

EXAMPLE 3 The process of this invention The same green pellet bed chargeas in Examples 1 and 2 was directly subjected from the beginning to hotgases at 1000 C., while the vacuum was maintained between and 60 mm.water column during only the first 3 minutes and then increased to from100 to 250 mm. Water column. After 26 minutes, including the 3 minutesheating-up period, the heating was shut off because the temperature ofthe pellets above the grate protective layer had reached the temperaturenecessary for hardening. After allowing the charge to stand for 5minutes without applying suction, the cooling with compressed air wasstarted and, as in Example 1, lasted 25 minutes.

With this method, it was possible to reduce the total period oftreatment by about 15 minutes and thereby attain an increase in outputfrom 14 to 18 tons of finished product per square meter grate surfacearea. Although the pellets were of the same quality as in Example 1, theheat consumption per kg. of burnt pellets necessary for the burning was700 kcal./ kg. burnt pellets, i.e. less by about Also the thermal stresson the grate bars was less, as they reached a maximum temperature of 550C.

EXAMPLE 4 As in Example 3, the green pellet bed charge was first treatedwith gases at 1000 C. The vacuum, which during the first 3 minutes wasbetween 40 and mm. water column, was then increased to 100 to 250 mm.water column, while the temperature of the hot gases was at the sametime increased to 1250 C. After a burning period of 19 minutes,including the3 minutes for heating up, the heating was shut off. Thetemperature above the protective grate layer had then reached 400 to 450C.

While the hood remained over the charge, atmospheric air from above theburner hood was sucked through the bed charge. As a result, the air,being in contact with the still hot burner hood, became heated to about400 to 500 C. The sucking in of preheated air was continued for a periodof 9 minutes. Consequently, the sensible heat in the upper layers of thecharge was carried into the lowermost layers and thus utilized for thehardburning thereof.

As compared with Example 3, further improvements were attained, namelythe output was now 20 tons per day per square meter grate area, the heatconsumption up to the end of the heating period was 500 kcaL/kg. burntpellets, i.e., lower by about 25%, as compared with Example 1 it was 60to lower, the temperature in the grate bars reached only 340 C., and thequality of the burnt pellets was still good by having a strength of 250to 350 kg. per pellet.

EXAMPLE 5 62 kg. of green pellets were charged to a height of 29 cm.onto the grate which was covered with a protective grate layer 10 cm. inthickness. Over this layer of green pellets, a layer of previouslyburned pellets ranging from 4 to 12 mm. in diameter was spread to athickness of 4 cm. The bed charge was then treated in the same manner asin Example 3; As compared with Example 3, the charge of green pelletswas more uniformly burnt. Particularly, the pellets directly below thecovering layer were of better quality. The heat consumption was 770kcaL/kg burnt pellets.

EXAMPLE 6 1000 C. for 3 minutes, followed by burning with gases at 1250C. until about 400 C. was reached above the protective grate layer, thencontinuation of the suction until pellet hardening temperature wasreached above the protective grate layer and then finally cooling withcompressed air. The heat consumption was 595 kcal./kg. 5 burnt pellets.

The operating data for the above examples is summerized in the followingtable for purpose of comparison.

as grate bars not heated above 600 C. will last for years. It is thusdemonstrated that the process of this invention employing an initialhigh temperature drying and preheating of the raw green pellets whileusing a low volume of gas flow is a practical advance in the art.

Having now described the means by which the objects of the invention areobtained, we claim:

A process for hard-burning raw green pellets not greater SINTER GRATEGREEN PELLET BED CHARGE Contents Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6

Grate Layer:

kgs 20 20 20 20 20 20 01115 10 10 10 10 10 10 Green Pellets:

kgs 70 70 70 70 62 62 33 33 33 33 29 29 -20 15-20 15-20 15-20 15-2015-20 28 28 28 28 28 28 2-3 2-3 2-3 2-3 2-3 2-3 Pellet Size, mms 2-102-10 2-10 2-10 2-10 2-10 Cover Layer:

kgs-.. 8 8 cms 4 4 Pellet Size, Innis 15-20 15-20 Total Charge, ems 4343 43 43 43 DURATION OF THE INDIVIDUAL PERIODS Drying, mins. 15 5Heating Up, min 13 3 3 3 3 Burning, nuns" 14 23 16 21 17 Standin mins. 55 5 Continued Suction, mins- 9 8 Cooling, mins 25 22 25 22 Total Time,mins 72 5 56 54 I 50 TEMPERATURES IN THE INDIVIDUAL PERIODS Drying, C300 1,000 Heating Up, 0-- 300-1, 100 1, 000 1, 000 1, 000 1, 000Burning, 1,250 1,250 1,250 1,250 1,250 Standing, C 1, 250 ContinuedSuction, C 1, 000-500 1, 000-500 ooling, C 20 20 20 20 20 QUANTITIES OFWASTE GAS CON SUMED IN THE INDIVIDUAL PERIODS Drying, Nmfi lmin. 7. 7 7.7 Heating Up, Nm. /min. 7. 5 3.3 3.3 3. 3 3.3 Burning, NrnA/min. 7. 54.2-4.0 4. 2-4.0 4.2-4.0 4 2-4.0 Standing, Nin /min!" Continued Suction,NmJ/min. 3. S 3. 8 3. 8 Cooling, Nmfi/min} 7. 5-7. 0 5. 5-6. 0 5. 5 5.5-6. 0 5. 5

VACUUM IN THE INDIVIDUAL PERIODS (OR PRESSURE DURING THE COOLING)Drying, mms/wat. 001. -100 200-300 Heating Up, mmsJwat. 00 -130 40-6040-60 40- 60 40-60 Burning, mms./wat. coll. 100-250 100-250 100-250100-250 Standing, nuns/Wat. col. 0 0 Continued Suction, mms./wat. col.-200 180-200 Cooling, mms./wat. col. 100-200 200-90 200-90 200-90 200-90RESULTS Output, Tons Per Day per sq. meter 18.0 20.4 16. 6 18.0 Strengthof Pellets, kg. per pe1let 300-350 250-350 300-400 300-350 Grate BarTemperature, C 550 340 510 400 Best Consumption without recovery,

Units per kg. pellets 1, 350 700 500 770 595 1 Normal cubic meters perminute.

1 Range of vacuums and pressure made to keep gas volume of flowsubstantially constant as porosity oi bed changed during heating andcoolin From the examples as detailed in the above table, it is seen thatExamples 3 to 6 of this invention achieved at least a one-third increasein output over the prior art process of Example 1. Pellets were ofacceptable strength, it being noted that pellets having a crushing 70strength of 100 kg. not to be shipped are acceptable, and pellets havinga strength of 250 kg. are shipable. The grate bar temperatures areimportant as cast iron grate bars heated to above 732 C. have a life ofbut about from two to three months in a sintering apparatus, where- 75than about 20 mm. in diameter and having a moisture content of about 9%H 0 and composed of fine-grained iron ore comprising forming a bed ofpellets at rest upon a horizontally moving sinter grate, drawing hot gasat a temperature of from about 1000 to 1400" C. through said bed with alow velocity corresponding to a vacuum of from about 20-60 min. Watercolumn to dry and preheat the pellets in the top of said bed to aboutthe temperature of said hot gases without bursting the pellets and thenincreasing the vacuum to about 70-200 mm. water 7 column to increase thehot gas velocity to continue the heating of the pellets throughout theentire bed and thus burn and heat harden the unbroken pellets at atemperature of from about 1000 to 1400 C.

References Cited'in the file of this patent UNITED STATES PATENTS2,191,911 Greenawalt Feb. 27, 1940 8 7 Lloyd July 10, 1945 Burrow et a1.Mar. 16, 1954 V Lellep June 12, 1956 Nora Apr 23, 1957 FOREIGN PATENTSGreat Britain Feb. 26, 1925 Great Britain June 20, 1956 Canada Nov. 18,1958

